A reciprocating engine, such as an internal combustion engine, is an engine that uses one or more reciprocating pistons to convert pressure into a rotating motion. In a typical example, a piston is housed in a sealable piston chamber or pressure chamber, and attached at its base to a rotatable shaft. As the piston slides along the piston chamber, the rotatable shaft is rotated, and vice versa. One example of a combustion cycle for the piston can include four piston strokes: intake stroke, compression stroke, combustion stroke, and exhaust stroke.
During the intake stroke, the piston is pulled out of the compression chamber, creating a vacuum, which draw in air from a sealable intake valve. Once the piston reaches the lowest point of its intake stroke, the intake valve is sealed, and the piston begins an upward compression stroke. The compression stroke slides the piston into the pressure chamber compressing the air. A combustible fuel can be added to the intake air prior to the intake stroke, or can be added during the compression stroke. At the end of the compression stroke, the air/fuel mixture is compressed in the pressure chamber until the mixture is combusted.
Combustion can occur due to the pressurized air/fuel mixture, or due to external ignition, such as a spark in the pressure chamber generated by a spark plug. During the combustion stroke, the explosion of the air/fuel mixture generates heat in the compressed gases, and the resulting expansion of the gases drives the piston away from the pressure chamber. Following the combustion stroke, a sealable outlet valve opens, and the piston is driven into the pressure chamber to push the combusted, or exhaust gases, out of the pressure chamber. The cycle of the combustion engine can then repeat.
Liquids in the cylinder can be problematic because liquids are relatively incompressible and when located in a combustion chamber where the fluids of combustion (air and fuel vapor) is normally compressed leads to a problem commonly known as hydrostatic lock. Hydrostatic lock occurs when a volume of liquid greater than the volume of the cylinder at its minimum (end of the piston's stroke) enters the cylinder. Since most common liquids are incompressible the piston cannot complete its travel; either the engine must stop rotating or a mechanical failure occurs ultimately resulting in engine damage upon starting of the engine during a hydrostatic lock condition.
An air turbine starter (ATS) can be used to initiate the rotation of the engine. The ATS is often mounted near the engine and can be coupled to a high pressure fluid source, such as compressed air, which impinges upon a turbine wheel in the ATS causing it to rotate at a relatively high rate of speed. The ATS includes an output shaft that is coupled to the turbine wheel, typically through a reducing gear box, to the engine. The output shaft thus rotates with the turbine wheel. This rotation in turn causes the engine to begin rotating. If a cylinder fills with liquid while the engine is off, the engine will refuse to turn when a starting cycle is attempted and this can damage the starter or engine.